Love this! Much better than canon, much more clever and ... Snape-like. I think that's the word I'm looking for. Really, if JKR wanted us to believe he was a spy par excellence who had all kinds of wicked magic, could he not see Voldemort's 'solution' coming? I like that it's Nagini who steps up here. She deserved a better ending than JKR gave her. Plus, the magic you've incorporated is beautifully creative.